Epoxy resins have excellent mechanical characteristics, and therefore, they are used in various industrial applications including coating compositions, adhesives, electric and electronic information materials, and advanced composite materials. Epoxy resins are widely used particularly in fiber reinforced composite materials composed mainly of a reinforcement fiber, such as carbon fiber, glass fiber, and aramid fiber, combined with a matrix resin.
Use of a prepreg produced by impregnating an epoxy resin in a carbon fiber substrate is popular in the production of fiber reinforced composite materials. The prepreg is laminated or preformed, and then heated to cure the epoxy resin, thereby producing a molded article. The property primarily required for a prepreg is the ability to provide a molded article exhibiting good mechanical characteristics, and recently in particular, other characteristics such as high productivity, namely, high curing speed are also required. This trend is prominent in industrial applications such as automobile applications where a high productivity is strongly needed.
In addition, currently available prepregs are required to be reactive at room temperature and normally freezable for storage. This requires preliminary arrangement of a freezer installation and thawing before use, and accordingly, there are demands for prepregs having improved storage stability to enable storage and easy handling at room temperature.
As techniques for improving storage stability, Patent Document 1 discloses a method for coating the surface of size-controlled imidazole derivative particles with a boric ester compound, and also describes that both high storage stability and curability can be realized simultaneously.
Patent document 2 describes that an epoxy resin composition with long term storage stability can be obtained by controlling the hydrolysable chlorine content in the epoxy resin in an appropriate range.
Patent Document 3 discloses a method designed to control the time period from the curing start temperature until reaching a certain degree of curing and adopt a curing agent having a limited particle size and a limited curing start temperature, and describes that both high storage stability and high curing speed were realized simultaneously.